Placebo response

What is placebo response?

Placebo effects in pharmaceutical trials vary widely, with response rates varying from 20% to 70%. The placebo response can include improvement in symptoms and even adverse reactions that have been associated with the antipsychotic being tested. Placebo effects can substantially influence conclusions about the efficacy of antipsychotic medications as they reduce any differences in response to the antipsychotic and the placebo.

What is the evidence for placebo response?

Moderate to high quality evidence suggests there is a significant positive placebo response over 12 weeks, which may be most apparent in studies with more efficacious drugs, younger samples, shorter illness duration, more severe baseline symptoms, shorter study duration, increased number of study sites, and non-university or non-Veteran Affairs settings. However, there is greater improvement in people taking antipsychotics than placebo.

Moderate quality evidence suggests there is greater improvement in symptoms from baseline to follow-up in placebo arms of clinical trials over time (1960 to 2014). Conversely, there is less improvement in symptoms from baseline to follow-up in treatment arms of clinical trials over time. This may be explained by enrolment of less severely ill patients at baseline in more recent studies, thereby decreasing the treatment effect and greater expectations in more recent studies that medications will improve symptoms, thereby increasing the placebo effect.

The evidence shows that there was greater improvement in symptoms in the placebo arm of studies using last observation carried forward (LOCF) methods to analyse data than in studies using mixed-effect models for repeated measures (MMRM). Studies involving more countries and those in outpatient settings had greater placebo response in the analysis of MMRM methods, while studies with shorter study duration showed greater placebo response in the analysis of LOCF methods.

October 2019

Last updated at: 12:28 am, 16th October 2019
To view documentation related to this topic download the files below
Fact Sheet Technical Commentary

NeuRA Libraries

Title Colour Legend:
Green - Topic summary is available.
Orange - Topic summary is being compiled.
Red - Topic summary has no current systematic review available.